Earlier we have reported that cGMP is involved as a mediator in the stimulus-secretion coupling in pancreas and flux of calcium is intimately linked with this process. In this study we have demonstrated that calcium can activate guanylate cyclase in a cell-free system. It has been found by immunocytochemical technique that cGMP is localized in the lumen, apical plasmalemma membrane and zymogen granules and possibly secreted by a process of exocytosis. Localization of cAMP in cytoplasm of acinar cell did not change significantly. From these studies it appears that calcium may be the mediator for the increase in cGMP and that cGMP along with Ca ions may participate in the process of exocytosis. BIBLIOGRAPHIC REFERENCES: Kapoor, C.L. and Krishna, G.: Hormone-induced cyclic GMP secretion from guinea pig pancreatic lobules. Science 196: 1003-1005, 1977. Krishna, G., Krishnan, N., Fletcher, R.T., and Chader, G: Effects of light on cyclic GMP metabolism in retinal photoreceptors. J. Neurochemistry 27: 717-722, 1976.